1. Field of the Invention
This invention relates in general to flexible sheet edging and anchoring means and in particular to clamping strips for anchoring screen fabric to locking channels on screen printing frames.
2. Background Information
U.S. Pat. No. 2,597,401 by Swanson, U.S. Pat. No. 3,225,407 by Daniels, U.S. Pat. No. 3,524,491 by Olson, U.S. Pat. No. 3,805,873 by Bloomfield, U.S. Pat. No. 3,982,306 by Curry, U.S. Pat. No. 4,631,786 by Curry, and U.S. Pat. No. 5,039,246 by Woodruff et al describe various channels and insertion strips for anchoring fabric panels, flexible sheets, webbing, covers, film, window screens and the like to walls, buildings, boat tops, swimming pools, porches, and various other objects. Although the object of these inventions is not specifically to anchor screen printing fabric to a frame, these inventions address the need to anchor a flexible sheet-like material to a structure. All use channels and insertable strips.
U.S. Pat. No. 3,489,085 by Kirkpatrick shows a channel and clamping strip for anchoring a rubber blanket around an offset printing cylinder.
In U.S. Pat. No. 3,482,343 by Hamu, U.S. Pat. No. 3,601,912 by Dubbs, U.S. Pat. No. 4,525,909 by Don Newman, and U.S. Pat. No. 4,539,734 by Messerschrmidt various channel and clamping strip anchoring means are described for screen-printing screens. In these inventions a screen fabric is anchored to a printing frame.
There are many examples in this art of edging strips that are not attached or bonded to screen fabric such as Messerschmidt, Daniels, Curry, and Woodruff. A precisely predetermined pattern of attached screen clamping strips imparts many beneficial qualities to a screen when it is stretched on a frame. Of the above art, only Kirkpatrick and Bloomfield teach sheet-like materials with attached clamping strips.
The clamping strips of printing screens, ideally, should be somewhat shorter than the lengths of each screen edge so as not to attach to the fabric in the corners. The channels of
Kirkpatrick and Bloomfield permit the insertion of the strips into the channel from above along the length of the channel as well as from the ends of the channel. It is necessary to enter the channel from above when anchoring a rectangular screen to a four sided frame. U.S.. Pat. No. 5,443,003 by Larson, and U.S. Pat. No. 5,522,314 by Eugene Newman teach permanently attached clamping strips to screens for insertion into a channel from above. These clamping strips, however, differ from the Kirkpatrick and Bloomfield inventions because they merely hook over one of the overturned lips defining a wall portion of the channel and are not really lodged within the channel. Bloomfield, as will be explained in a later paragraph, describes a locking mechanism that becomes stronger as more stress is applied. This characteristic differentiates Bloomfield from Kirkpatrick as well as from Newman and Larson whose attachments are more likely to weaken as more stress is applied. If the teaching of Bloomfield were to be combined with Newman and Larson, what results is an advancement in the art of locking attached clamping strips along a screen edging into channels on all four sides of a screen frame. However, as will be explained in a later paragraph, this combination, if advanced, has a deficiency. The present invention overcomes this deficiency.
In Kirkpatrick and Bloomfield one edge of a substantially flat clamping strip enters into a relatively deep undercut that forms a part of one side of the channel, with the clamping strip so inserted into this undercut, the clamping strip is then redirected so as to insert the opposite edge of the clamping strip into a relatively shallow undercut that forms a part of the opposite side of the channel. This edge of the clamping strip is inserted all the way into this shallow undercut until it abuts against the inside wall. With a clamping strip that is wider than the combined width of the shallow undercut plus the width of the opening between the undercuts but narrower than the overall width of the channel which would include both undercuts and the opening between the undercuts, the clamping strip will extend all the way into the shallow undercut, across the opening between the undercuts, and partially into the deeper undercut. With the flexible sheet tucked under and around the clamping strip so that it is between the clamping strip and the walls of the undercut, the stronger the pull of the flexible sheet the more firmly the clamping strip presses the flexible sheet against the wall within the channel. The locking action increases in direct relation to the increase in stress. The Bloomfield channel and clamping strip anchoring device, however, will not work with a symmetrical channel having two equally deep undercuts. The only way to lodge the Bloomfield clamping strip in such a channel is through the ends of the channel. In a four sided frame, it is impossible to insert perpendicular edging strips attached to a flexible sheet through the ends of the channels on the sides of the frame in both the X and Y axis. Most roller frames used in the screen industry have symmetrical undercuts in the channels. Therefore, the Bloomfield invention is useless with these frames.
Dubbs and Hamu show unattached clamping strips and a single undercut channel in which the strip abutment is against a flat wall rather than a shallow undercut. The clamping strip of Dubbs and Hamu will not lock into a symmetrically shaped channel.
Given all of the above recited art, there are no combinations of technology in which clamping strips attached to the edges of screen fabric can be locked onto a printing frame with symmetrically shaped locking channels. U.S. Pat. No. 4,525,909 by Don Newman describes a symmetrical channel on a roller frame for screen printing. This invention describes unattached clamping strips that must be inserted from the roller ends. This type of roller frame with its symmetrical locking channels has become very popular and widely used in the screen printing industry. With their large numbers, a significant problem is how to use these frames with clamping strips attached to the edges of screen fabric. An object of this invention is to solve this problem. Another object of this invention is to provide an attached screen fabric clamping strip capable of locking screen fabric into a greater variety of frame locking channels.